1. Field of Invention
The present invention relates to improved sulfurized oils and to methods for making the same. More particularly, this invention relates to a process for producing a sulfurized oil for extreme pressure lubrication and metal cutting processes.
2. Description of the Prior Art
This invention relates to an improved sulfur-containing lubricant suitable for use in preparing lubricating oil compositions having improved extreme pressure properties for metalworking purposes.
Mineral lubricating oils containing sulfurized addition agents have been extensively used as metalworking lubricants and especially as cutting oils. In recent years, it has been found that the mineral oil component of such lubricants contain constituents that may be toxic or carcinogenic in nature and therefore undesirable due to the potential harm that might occur when brought in contact with the human body. It has been found that the polycyclic aromatic constituents naturally present in most mineral lubricating oils are the undesirable constituents. Accordingly, it is necessary to remove these constituents from the oil in order to have a mineral lubricating oil suitable for the preparation of metalworking oils that are not harmful to the human body. The polycyclic aromatics are highly polar and can be removed by suitable solvent extraction methods well known to the art. Naturally occurring paraffinic or naphthenic petroleum oils or solvent extracted oils contain less than about 10 percent carbon in aromatic bonding, as determined by the n-d-M Method used for determining the hydrocarbon type analysis of lubricating ols, have been found to be relatively free of toxicity and carcinogenicity characteristics. A description of the n-d-M Method may be found in "Aspects of the Constitution of Mineral Oils" by K. Van Nes and H. A. Van Westen, Elsevier Publishing Co., Inc., New York, 1951. Such oils containing minor amounts of the polar aromatic constituents are relatively poor vehicles for retaining sulfur in an active form. Such oils are now used as the principal component of commercial metalworking lubricants.
It has been known for many years that sulfur increased load bearing capacity of oils for lubrication of hypoid and other gears where extreme pressures may be encountered. In metalworking and the like, many such oils have been and are now used for metal cutting purposes. In some cases the mineral oils have been sulfurized directly. In other compositions, materials such as lard and fatty oils are sulfurized and are added to the mineral oil base. Both types of the products serve the purpose but at the same time have disadvantages. It is difficult to sulfurize directly the highly refined base oils needed to avoid toxicological problems. The amount of sulfur that can be incorporated is insufficient to provide satisfactory performance and frequently sulfur deposits out during storage. Sulfurized fatty oils do not perform satisfactorily in heavy duty machining operations. Free sulfur containing mineral oils commonly contain generally less than 2.0% of sulfur in a stable composition and must be controlled to a narrow range of free sulfur to maximize sulfur concentration and sulfur stability. In processes currently used difficulties in controlling reaction rates and sulfur content are encountered. Some of these difficulties arise in the need to closely monitor the sulfur content of the mineral oil during long reaction time by withdrawing samples during reaction. Poor control over sulfur content of the final product is due to the difficulty in obtaining rapid analyses needed to predict final sulfur content.
The degree of refining that a mineral oil receives as well as its origin, hence composition, influences the ease of incorporation of free sulfur. Thus, unextracted oils containing aromatics such as mid-continent distillates need to be heated only to about 300.degree.-320.degree. F. to incorporate appreciable, up to (about 1.5%, free sulfur in a stable form. On the other hand, it is quite difficult to produce appreciable free-sulfur containing oils from mid-continent mineral oils from which aromatics have been extracted.
The present invention is directed to a process which produces appreciable free sulfur containing oils from highly refined extracted base oils. More specifically, the present invention is directed to a process in which a sulfurized oil which contains less than about 0.5% uncombined sulfur and about 0.1-5.0% combined sulfur is used to stabilize elemental sulfur to produce a mineral oil containing about 1.0 to about 2.0% of uncombined sulfur. Combined sulfur is sulfur which when contacted with the oil reacts with the oil and becomes chemically bound by the oil molecules. Uncombined sulfur is sulfur which when heated with stabilizing oil is held in solution, and is not chemically bound by the oil molecules. The uncombined unreacted sulfur is the component of the oil which provides the extreme pressure and metal cutting lubricant properties. The disclosed process has short reaction times and is convenient to use. It requires little analytical monitoring and produces a sulfurized mineral oil with a controlled sulfur content. The sulfurized oil product has excellent extreme pressure, load bearing and metal working properties. Thus, it is an object of this invention to produce a novel improved sulfurizing process for lubricating oils. Another object of the invention is to produce a sulfur solubilizing mineral oil composition. Still another object of the invention is to produce a novel sulfurized lubricating oil which provides superior lubrication in high pressure and metal working environments. A further object of the invention is to produce a sulfurized mineral oil which is very stable. Another object is to provide a process more suitable for carrying out as a continuous process. Patents which claim processes producing sulfurized mineral oils are U.S. Pat. No. 1,824,523 and U.S. Pat. No. 2,222,643. The U.S. Pat. No. 1,824,523 claims the method of producing a sulfurized cutting oil which comprises incorporating into a hydrocarbon oil free from fatty oil and containing initially at least 0.75% sulfur additional sulfur sufficient to bring the total sulfur content of the oil to at least 2%. U.S. Pat. No. 2,222,643 claims a composition of matter comprising sulfurized monohydric aromatic extract of a mineral lubricating oil fraction, said sulfurized extract being prepared by such acting monohydric aromatic extract to the action of elemental sulfur in an amount between about 1% and about 10% by weight at a temperature between about 300.degree. F. and 375.degree. F., followed by cooling to below 250.degree. F.
Free sulfur has limited solubility in refined mineral oils. The maximum solubility is about 0.4%. A solvent or solubilizing material is needed to be present in the oil to solubilize and stabilize sulfur. Very effective sulfur solubilizing products are achieved when the mineral oil itself is converted to such a material. To produce a solubilizing oil a mineral oil may be retracted with sufficient sulfur to give a sulfurized oil which will solubilize enough free sulfur to give extreme pressure properties to the lubricant oil.